About Forty Last-Glacial Lake Missoula Jökulhlaups through Southern Washington

Abstract

The rhythmic Touchet Beds in the Walla Walla and lower Yakima valleys resulted from many separate backfloodings by hydraulically ponded glacial Lake Missoula water. At least once this episodic lake briefly contained half the of water that catastrophically drained the largest glacial Lakes Missoula. Evidence that the Touchet Beds rhythmites originated from brief backfloodings includes up-valley thinning and fining of locally derived bedload, upvalley paleocurrents, and upvalley transport of erratics derived from Cordilleran ice. Evidence that a lengthy nonflood environment followed the emplacement of each of about 40 Touchet Beds rhythmite includes inferred eolian and slopewash sediment overlying many rhythmites, uncontaminated Mount St. Helens "set S" tephra couplet atop one rhythmite as much as 220 m below the maximum level of backflooding, filled semiconsolidated rodent burrows throughout the 30 m of the thickest section, and dispersed skeletons of mammals. The lack of weathering or soil within the Touchet Beds suggests that all rhythmites are late Wisconsin. Bottom sediment of glacial Lake Missoula in Montana consists of rhythmites each interpreted as the record of a gradually deepening lake. Forty superposed rhythmites record about 40 late-Wisconsin fillings and emptyings of glacial Lake Missoula. The complementary records of about 40 separate glacial Lakes Missoula and about 40 great floods in southern Washington and in the Willamette Valley, Oregon indicate that the Missoula floods were great jokulhlaups. The last several floods were smaller than earlier ones because the controlling dam of Cordilleran ice thinned during deglaciation.